Hydraulic drive

ABSTRACT

A hydraulic drive for a hydraulic load or for a hydraulic system includes a variable-displacement pump which is mechanically coupled to a variable-speed drive motor. The hydraulic drive also includes a control configured to adjust at least the capacity of the pump and optionally the speed of the drive motor. The hydraulic drive also includes a torque detecting device configured to detect or calculate a current output torque of the drive motor. Based on the current output torque of the drive motor, the capacity of the pump is set to a value at which the current output torque is at or below a set maximum value.

The present invention relates to a hydraulic drive for a hydraulic consumer in accordance with the preamble of claim 1.

A hydraulic drive of this generic type is known from the prior art in accordance with EP 1 387 090 A2. This document discloses the arrangement of a variable speed electro-motor that drives in this case two constant volume pumps. At least one of the pumps can be switched on and off in dependence upon the prevailing consumer pressure by virtue of the fact that this pump that preferably comprises the greater stroke volume is switched as desired into the circulation mode. It is possible in this manner to embody the motorized drive with a comparatively small torque.

EP 0 805 922 B1 discloses a hydraulic pump control system of an aircraft, which system uses a variable displacement hydro-pump that is driven by a variable speed electro-motor. The rotation speed of the motor is regulated in such a manner that the rate at which its speed is reduced when the system requires less pressure is smaller than the rate at which its speed is increased when the system requires more pressure. In other words, for example, as the demand for pressure reduces, the stroke volume of the pump is initially reduced, whereas as the demand for pressure increases the motor rotation speed initially increases.

EP 464 286 B1 is known as further prior art and this likewise discloses an electro-hydraulic drive unit. This drive unit uses a variable speed electro-motor to drive a variable displacement pump. The variable displacement pump is adjusted by means of a spring to a position that provides the maximum stroke volume, from which position said pump is adjusted in the direction towards the minimum stroke volume if the pressure in the hydraulic system rises above a predetermined pressure value.

DE 10 2007 007 005 A1 discloses finally an electro-hydraulic control arrangement having a variable displacement pump that is driven by a variable speed electro-motor. In order to control the pump a two-position regulator is provided that switches the pump to a maximum or minimum conveying position and in fact in dependence upon whether a pressure threshold value is exceeded or is not achieved.

On the basis of the aforementioned prior art, it is an object of the present invention to provide a hydraulic drive for a hydraulic system that at least comprises a variable displacement (variable stroke volume) pressure medium source (hydro-pump) and a variable speed drive motor, wherein the drive comprises improved properties with respect to the prior art. One object is to render the hydraulic drive more cost-effective. A further object is to reduce the expenditure with regard to the control technology. Finally, it is an object of the present invention in accordance with the construction principle of “downsizing” in particular to reduce the power of the drive motor without adversely affecting the functioning reliability of the downstream hydraulic system.

This object is achieved by virtue of a hydraulic drive having the features of claim 1. Advantageous embodiments of the invention are the subject matter of the subordinate claims.

The invention therefore relates to a hydraulic drive for one/several consumer(s) or for a hydraulic system having a variable displacement pump that is mechanically coupled to a variable speed drive motor and a control for adjusting the stroke volume of the pump. The pump is torque-controlled in accordance with the invention. In other words, in order to drive the pump a predetermined torque is not to be exceeded. The torque regulator does not engage as long as the torque is below the predetermined torque even in the case of the maximum stroke volume, which torque is the product of the system pressure and the stroke volume, described as the quantity of pressure medium that is conveyed by the pump per rotation. If the pressure that is determined by the loading or can be predetermined by means of a pressure valve in the case of acceleration procedures and in the case of a loading on the stop becomes so high that the product of the pressure and stroke volume achieves the predetermined value, then the pump is adjusted in the direction of a smaller stroke volume. The product of the pressure and the stroke volume remains at least almost constant.

It is possible in this case to use a drive motor that has a relatively small maximum torque, as a consequence of which the drive is overall more cost-effective and also is more economical to run as a result of the reduced power consumption of the motor.

The term “torque sensing device or torque detecting device” is understood in this case to include any device that is suitable for drawing conclusions relating to the prevailing torque loading of the drive motor. This device can, for example, be a torque sensor that is positioned on the drive motor and/or its driven shaft or it can be a current measuring device that measures the prevailing current consumption of the motor and determines therefrom the torque that is being output by the motor. It is also possible to measure the prevailing pressure in the hydraulic system that is downstream of the pump and to adjust the stroke volume in a controlled or regulated manner in such a manner that the predetermined maximum torque is not exceeded.

A torque regulator that is known per se for a variable displacement pump is particularly preferred as a “torque sensing device or torque detecting device”, wherein a valve piston of a regulating valve is influenced in one adjustment direction by a resilient force that defines the maximum torque and said valve piston is influenced in the other adjustment direction by a force that is produced on a lever from the point of contact of a piston that is influenced by the pump pressure, said point of contact being dependent upon the stroke volume.

The drive motor is a variable speed motor. It follows from this that if the stroke volume of the pump is reduced in order to limit the necessary torque at the motor, then this would lead to a reduced speed of the hydraulic movement of one or several consumers (for example a lift cylinder). If the intention is to retain the movement, then the rotation speed of the drive motor can be increased as the stroke volume of the pump reduces, as a consequence of which the volume flow produced by the pump that has been pivoted backwards increases in dependence upon the rotation speed without the output torque increasing.

However, if the hydraulic consumer is at a dead stop, when the pressure increases it is only necessary to maintain one pressure so that the rotation speed of the drive motor does not need to be increased, on the contrary rather said rotation speed is to be reduced.

The power consumption of the motor can be maintained below a fundamentally (quasi) constant value by directly or indirectly controlling the pressure medium source in dependence upon the torque, in such a manner that the motor can be of a smaller design with respect to its maximum possible power output.

The invention is explained further hereinunder with the aid of a preferred exemplary embodiment with reference to the accompanying figures, in which:

FIG. 1 illustrates in comparison: a hydraulic drive having two individually driven pumps having a small stroke volume in accordance with the prior art, a hydraulic drive having a permanent and a switchable constant volume pump in accordance with the prior art and a hydraulic drive having a torque-controlled, variable displacement pump and a variable speed drive motor in accordance with a preferred exemplary embodiment of the invention and

FIG. 2 illustrates a hydraulic regulation/control for a pressure medium source, in particular a variable displacement pump in accordance with the invention.

In accordance with the attached FIG. 1, an already known hydraulic drive is illustrated in the left illustration, which hydraulic drive uses two independently driven constant volume pumps. Each pump is thus driven by a dedicated and if necessary variable speed motor. This construction is comparatively complex and costly. Moreover, said construction requires a greater amount of installation space which rather renders this solution uneconomic.

The likewise known hydraulic drive in accordance with the illustration in the middle in the figure is an improvement. This drive also uses two constant volume pumps that are however driven simultaneously by a single motor. One of the constant volume pumps can be switched as desired directly to a tank by way of a valve, in this case a 2-way, 2-position switching valve having an open position and a closed position, as a consequence of which this pump is removed from the downstream hydraulic system. It is possible in this manner to reduce the load on the motor.

The illustration on the right in accordance with the attached figure illustrates on the other hand a hydraulic drive 1 in accordance with the present invention.

Accordingly, only one single pump 2 is provided, which pump is embodied as a variable displacement pump. For example, the pump 2 can be embodied as a swash plate pump, wherein the angle position of the swash plate of said pump can be varied continuously or in steps. This pump 2 is connected on the output side to a hydraulic system 4 that comprises for example an actuating valve by means of which a consumer 8, in this case a piston-cylinder unit, can be actuated.

The single variable displacement pump 2 is driven by a single, preferably variable speed drive motor 10, for example an electro-motor, which electro-motor is mechanically coupled by way of a drive shaft to the pump 2.

Finally, a control unit 11 is provided for the drive motor 10 to which are connected in this case at least one torque sensor and a system pressure sensor and a rotation speed sensor and a sensor for sensing the prevailing set stroke volume (angle position of the swash plate) of the pump 2. The control unit 11 is programmed or can be programmed in such a manner that the hydraulic drive 1 builds up and maintains a predetermined (adjustable) consumer or system pressure.

In accordance with the invention, a comparatively less powerful drive motor 10 is constructed, which motor renders possible a small maximum output torque. Therefore, as this maximum output torque is achieved or slightly less than said maximum output torque is achieved, for example in the event of a pressure increase in the hydraulic system, the variable displacement pump 2 is moved backwards, i.e. its delivery capacity is reduced, as a consequence of which the prevailing output torque of the drive motor 10 is reduced. In this manner, the pump 2 and/or its stroke volume can be adjusted in such a manner that irrespective of the pressure level in the downstream hydraulic system the prevailing output torque of the motor 10 does not exceed or remains less than a maximum torque value. In the most favorable case, the output torque can be held fundamentally constant. In other words, the equation P×V=const=T applies, wherein:

P: is the pressure in the hydraulic system

V: is the stroke volume of the pump

T: is the driving torque of the motor.

If the delivery capacity of the pump 2 is reduced in this manner, the hydraulic pressure prevailing at the consumer consequently reduces and/or the hydraulically activated movement of a consumer slows down. In order to avoid this, it is provided as an option in accordance with the preferred exemplary embodiment of the invention for the drive motor 10 to be embodied as a variable speed motor. Therefore, as the delivery capacity of the pump 2 reduces, the rotation speed of the motor 10 can be increased for the purpose of maintaining constant the output torque in order to maintain the required prevailing consumer pressure and/or the movement rate of the consumer.

Reference is made to the circuit diagram as shown in FIG. 2 with regard to the control of a pressure medium source of this type and/or a variable displacement pump 2, which circuit diagram illustrates the pressure medium pump in accordance with the present invention. It is evident in this figure that the housing 12 accommodates the hydraulic drive that comprises a cylinder drum 40, a drive shaft 22, a swash plate 43, an outwards-pivoting piston 75 that defines a control chamber 101, a restoring spring 85 on the outwards-pivoting piston 75 and said hydraulic drive further comprises an inwards-pivoting piston 76 that defines a control chamber 102. A high pressure duct 103 and a low pressure duct or intake duct 104 are routed in the housing 12. The control chamber 101 is permanently connected by way of a duct 105 to a high pressure duct 103. A regulating valve 80 is installed on the housing 12. Said regulating valve comprises a torque regulating part valve 106 and a pressure regulating part valve 107 that in the idle position by way of a first inlet and its regulating output switches a regulating output of the torque regulating part valve 106 through to a control line 108 that leads to the control chamber 102 at the inwards-pivoting piston 76. A second input of the pressure regulating part valve 107 is connected to the high pressure duct 103. An input of the torque regulating part valve 106 is likewise connected to the high pressure duct 103, whereas a second input of this torque regulating part valve 106 is open to the inner chamber of the housing 12 that is at tank pressure. A regulating piston of the pressure regulating part valve 107 is influenced in terms of reducing the pivot angle of the swash plate 43 by the pressure in the high pressure line 103 and in terms of the opposite direction by an adjustable spring.

A two-arm lever 115 is mounted in the housing 95 of the valve 80 and a piston 116 engages one lever arm of said lever, which piston is guided in the housing of a return element and which piston is influenced by way of the duct 105, the control chamber 101 and the bore 92 in the outwards-pivoting piston 75 by the pressure in the high pressure duct 103. The distance of the point of engagement changes with the pivot angle of the swash plate 43. The other arm of the lever 115 is located between one end of the regulating piston of the torque regulating part valve 106 and an at least almost opposite-lying adjustable spring 117 that is engaging the lever arm. Furthermore, the regulating piston of the torque regulating part valve 106 is influenced in the direction towards the lever arm by an adjustable spring 118. The spring 117 and the spring 118 that is adjusted such that it is weaker than the spring 117 generate a fixed amount of torque in one direction at the lever 115. With the aid of the effective surface of the piston 116 the high pressure in the duct 103 generates at the lever 115 a torque that is not fixed like said fixed torque and depends upon the position of the outwards-pivoting piston 75 or in general upon the pivot angle of the swash plate 43. In the case of a predetermined pressure it is only possible to maintain the equilibrium between the torques generated by the two springs in the case of a predetermined pivot angle. If the equilibrium is disturbed as a result of a change in pressure, the valve piston of the torque regulating part valve 106 is moved out of its regulating position in such a manner that pressure medium flows into the control chamber 102 or pressure medium can flow out of the control chamber 102 until a different pivot angle is achieved at which the equilibrium between the torques at the lever 115 is restored.

Thus, if the hydraulic system pressure increases and consequently the pressure in the line 103 increases, the valve piston of the torque regulating part valve 106 as shown in FIG. 2 is moved towards the right, as a consequence of which the system pressure is applied by way of the line 108 to the inwards-pivoting piston 76. This therefore reduces the angle position of the swash plate 43 until the torque at the lever arm 115 has achieved the equilibrium. The drive torque that is to be output by the motor onto the shaft 22 can consequently be maintained at a constant level. 

1. A hydraulic drive for a hydraulic consumer or a hydraulic system comprising: a variable displacement pump mechanically coupled to a variable speed drive motor; and a control configured to adjust a stroke volume of the variable displacement pump, wherein the variable displacement pump is configured to be torque-controlled.
 2. The hydraulic drive as claimed in claim 1, further comprising: a pressure sensing device configured to sense a prevailing pressure at the hydraulic consumer or in the hydraulic system, wherein the variable displacement pump is configured to be adjusted such that a predetermined value of an output torque of the drive motor is equivalent to a constant value which is equivalent to the consumer/hydraulic system pressure multiplied by the stroke volume of the variable displacement pump.
 3. The hydraulic drive as claimed in claim 2, wherein the variable displacement pump is equipped with a mechanical-hydraulic torque regulator.
 4. The hydraulic drive as claimed in claim 1, wherein the drive motor is configured such that a rotation speed thereof is adjusted based upon a prevailing stroke volume of the variable displacement pump such that a predetermined movement rate of the hydraulic consumer is maintained. 